How Does a Jet Engine Start?

Easier to start when compared to its propeller engine counterparts, jet engines only require compressed air and fuel to start. Jet engines are typically recognized for being a type of reaction engine that generates thrusts by jet propulsion.

A jet engine works by sucking air through the intake, sending the air through a fan equipped with a row of blades that compresses the air. Then, a majority of the air travels through the engine. This air is called bypass air, and it is used for the engine’s thrust, and to cool the engine. A final portion of the air dissipates as it goes through the “hot section” of the engine.

As the air travels through the “hot section,” it passes through 14 rows of compressor blades that further compress it. At this point, the hot compressed air passes into the combustion chamber where it mixes with fuel and burns. Within a jet engine, the fuel-air mixture burns at a constant rate, allowing the air to heat up, expand, and make its way out of the turbine blades.

Produced by Embraer, the Embraer ERJ 145 family is a series of twin-engine jets that contain five rows, two of which are high pressure turbines, and three that are low pressure turbines. As air passes through the turbines, the turbines, shaft, and compressor blades begin spinning. Hot air starts to make its way through the turbines, subsequently leaving the engine, and generating more thrust.

In order to get the engine to run, the engine core needs to spin at 14% of its maximum speed before the igniters can kickstart. Engine core speed is defined as “N2,” and is expressed as a percentage of maximum RPM. In an ERJ, 100% of the N2 is approximately 16,000 RPM; thus, the engine requires at least 2,200 RPM before the igniters can begin firing. That being said, the core should reach 28.5% of the N2, roughly a little over 4,500 RPM before the engine can introduce fuel and ignite the mixture.

Fuel is typically dispersed at 200 pounds per hour, which is about ½ a gallon per minute. To handle this much fuel, large amounts of compressed air is required in the combustion chamber. The most common source of compressed air includes an auxiliary power unit (APU).

Within a small turbine engine located near the tail of an aircraft, the APU provides compressed air and electricity. The auxiliary power unit is also tasked with powering an aircraft’s electrical systems on the ground, all the while serving as a backup generator for electrical and pneumatic requirements. In the case that the APU fails, you will require an external source of compressed air, the most common being a huffer cart. A huffer cart is an air compressor that can be mounted on the side of an aircraft, providing compressed air for starting an engine.

Lastly, air may also be sourced from a running engine. Turbine engines usually release bypass air, which can be distributed among multiple engines in an aircraft. If you open the bleeds on one engine, and open the crossbleed in another engine, your aircraft is ready to start.


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